1. Field Of The Invention
The present invention relates to an image forming apparatus, of which digital copy machines, laser printers, laser facsimiles and the like are representative. Specifically, the present invention relates to a measure for preventing the worsening of image quality caused by excessively charged developer (overcharged toner) that resides on a development sleeve (developer supply member) of a development apparatus.
2. Description Of The Related Art
Conventionally, as an image forming (printing) operation in an electrophotographic image forming apparatus such as a laser printer, an electrostatic latent image is formed on a photosensitive drum based on image data sent from a host apparatus such as a personal computer, toner (developer) is affixed to this electrostatic latent image, and image data is made as a development on the photosensitive drum. Afterwards, recording paper transported on a paper transport path is passed between the photosensitive drum and a transfer roller, and the toner image on the photosensitive drum is transferred to the front side of the recording paper. Then, this recording paper passes a fixing roller, and the toner image is fixed onto the recording paper with heat and pressure by the fixing roller.
As a configuration of the development apparatus in which image data is made as a development on the photosensitive drum, a development tank filled with toner and a development roller that supplies this toner to the photosensitive drum are provided. The surface portion of this development roller is made of metal and configured of a development sleeve that has been charged to a predetermined electric potential. As a developing operation by this development apparatus, the toner inside the development tank is carried on the surface of the development sleeve, and the toner is carried by the rotation of the development roller to a development region facing the photosensitive drum. In this development region, the toner on the surface of the development sleeve is adsorbed on the electrostatic latent image on the photosensitive drum, making this electrostatic latent image visible.
In recent years, the development of image forming apparatuses of this type provided with a double-sided printing function that performs printing on both the front side and the back side of the recording paper has been advancing. As a method for this double-sided printing, ordinarily printing is performed one sheet at a time with front and back print data. That is, a main transport path and a reverse transport path are provided as the transport path that transports the recording paper, and a switchback mechanism for performing switchback transport of the recording paper is also provided. After transporting the recording paper in the main transport path and performing front side printing, that recording paper is transported into the reverse transport path via the switchback mechanism, thus reversing the front and back of the recording paper. It is again transported into the main transport path, and printing is performed on the back side of the recording paper.
When performing double-sided printing with this type of image forming apparatus, as described above, after transferring a toner image to one side of one sheet of recording paper, a fixing process is performed that fixes the toner image to that recording paper, and afterwards, back side printing subsequently begins. Because heat fixing is generally performed in the fixing process, the moisture content of the surface of the recording paper used in this fixing process is reduced by the fixing heat. As a result, the surface resistance value of the recording paper when performing back side printing is increased in comparison to the surface resistance value of the recording paper when performing front side printing.
Ordinarily, a constant current control is performed in the transfer process that controls the transfer operation with a constant electric current, but as described above, when performing transfer for double-sided printing, when executing a transfer operation with a constant current control to identical paper of a different surface resistance value, the transfer voltage applied to the transfer roller when performing the transfer process is vastly different for front side printing and back side printing. The surface resistance value of the paper (ordinarily, about 1×106 to 1×1010Ω·cm) changes about 1×101 to 1×102Ω·cm depending on the type and the size of the paper, the moisture content of the paper, the surrounding environment, and the like, and due to the large change in the surface resistance value of the paper that accompanies the fixing process as described above, the transfer voltage when printing the back side may be an applied voltage nearly two times the transfer voltage when printing the front side.
When performing double-sided printing for multiple sheets of paper in such conditions, when printing the second and subsequent sheets, print defects are generated due to “fog”, which is described below. Following is an explanation of the circumstances in which this “fog” is generated.
FIG. 7(a) is a drawing that illustrates the change in the electric current of the transfer roller and the surface electric potential of the photosensitive drum in the transfer process of a conventional example. FIG. 7(a) shows the positional relationship between the transfer roller and the photosensitive drum. FIG. 7(b) shows the relationship between position and electric current in the transfer roller in this conventional transfer process. FIG. 7(c) shows the relationship between the position of the photosensitive drum and the surface electric potential of the photosensitive body in this conventional transfer process.
As shown in FIG. 7(a) through (c), the paper is sandwiched between a transfer roller 101 and a photosensitive body 102, and when transferring the toner image of the photosensitive body 102 to the paper, the transfer current i1a (electric current of the transfer roller) that flows in the direction of the photosensitive body in the transfer process for the front side of the first sheet of paper is about constant.
Thus, the surface electric potential v1 of the photosensitive body 102 after the transfer process for the front side of the first sheet of paper has been performed (immediately before performing the transfer process for the back side of the first sheet of paper) is approximately constant across the entire photosensitive body 102.
However, when performing the transfer process for the back side of the first sheet of paper, because the paper has been through the fixing process when printing the front side, as described above, the resistance value of the paper has increased, and because of that increase in the resistance value the transfer current cannot easily flow. As a method for eliminating such a problem, the constant current control system described above that always lets a constant current flow is adopted. This method attempts to maintain a constant current by increasing the voltage by the extent that it is difficult for the current to flow.
On the other hand, outside the region where paper is arranged (a portion separate from the paper passage region), paper does not lie between the photosensitive body 102 and the transfer roller 101, current flows easily because resistance is the same as under the condition when performing the transfer process for the front side of the first sheet, and in comparison to when performing the transfer process for the front side of the first sheet, a large amount of transfer current i1b flows on the photosensitive body 102 outside of the region where paper is arranged. Outside the region that paper is arranged, the voltage at this time is the same as for the region where paper is arranged (paper passage region), and is a higher voltage than when performing the transfer process to the front side of the first sheet.
Due to this phenomenon, on the photosensitive body 102 outside the region where the paper is arranged, a large amount of transfer current of a polarity opposite to the charging polarity of the photosensitive body flows at a high voltage (if this continues for a long time, it is possible that the photosensitive body will be damaged), and as a result the charging potential of the photosensitive body 102 decreases (a balancing phenomenon due to reverse potential occurs).
Thus, in the photosensitive body 102 after performing the transfer process for the back side of the first sheet of paper (immediately before performing transfer for the front side of the second sheet of paper), the surface electric potential v2 of both edges that are outside the region where the paper is arranged decreases.
On the other hand, on the development sleeve in the development apparatus, the toner affixed to the region corresponding to the paper size, that is, corresponding to the region where the paper is arranged, is taken (supplied to the photosensitive body) as necessary during the image forming operation. However, for toner affixed in the region corresponding to the region where paper is not arranged, which is the region outside the paper size, a state is constantly maintained wherein the toner is kept on the development sleeve, frictional charging due to friction with the photosensitive body 102, friction with a toner layer thickness regulating member (doctor blade), and the like is repeated, and the charging potential rises too much. That is, overcharged toner is always affixed on both sides in the axial direction (the region corresponding to the region where paper is not arranged).
FIG. 8 shows how toner t1 and t2 is affixed on the development sleeve 103. The density of the toner t1 and t2 in FIG. 8 indicates the charging quantity. Within a developer rising region R100 of the development sleeve 103, a central portion R101 not including the both side portions R102 in the axial direction corresponds to the image region. In this central portion R101, because an electrostatic latent image is formed on the photosensitive body 102 during the image forming operation and the toner t1 is stripped away, the toner t1 is periodically replaced (metabolized in the developing process), the charging quantity of the toner t1 does not increase too much. On the other hand, in both side portions R102 in the axial direction of the development sleeve 103, no electrostatic latent image is formed on the photosensitive body 102, and the toner t2 is not stripped away. Thus, rotation on the development sleeve 103 continues without the toner t2 being replaced, and the charging quantity is increased too much due to members that rub against the development sleeve 103. The toner t2 in this state is referred to as “overcharged toner”. This toner t2 is also referred to as “dead developer (dead toner)”, meaning that it does not contribute to image forming (is not consumed), and both side portions R102 of the development sleeve 103 are referred to as “dead developer generating regions”, meaning that this sort of dead developer is generated.
FIG. 9 shows the change in the charge amount of the toner t1 and t2 on the development sleeve 103. As seen from this diagram, the toner t1 in the central portion R101 of the development sleeve 103 contributes to development (is consumed), and its charging quantity c1 does not rise excessively. On the other hand, because the toner t2 in both side portions R102 of the development sleeve 103 does not contribute to development (is not consumed), its charging quantity c2 rises excessively, and becomes so-called “dead toner”.
In theory, the initial charging polarity and electric potential of the photosensitive body 102 are set assuming that all of the toner on the development sleeve 103 is charged to an appropriate level, and toner does not affix to the region outside of the electrostatic latent image. However, due to a decrease in the surface electric potential in the vicinity of both edges which are outside the paper arranging region of the photosensitive body 102 as described above, and overcharged toner affixing in both side portions R102 of the development sleeve 103, the phenomenon occurs that overcharged toner affixes unintentionally on the photosensitive body 102 outside the paper arranging region.
In this way, a state in which overcharged toner is affixed to the photosensitive body 102 continues until the photosensitive body completes at least one full turn (until the overcharged toner passes the cleaning apparatus and is eliminated). In such a state in which overcharged toner is affixed, when the second page of paper is transported onto the photosensitive body 102, if a shift of that paper transport position occurs in the axial direction of the photosensitive body (displacement in the width direction of the paper), an image fogged by overcharged toner is transferred onto this paper due to one edge of the paper making contact with the region where overcharged toner is affixed, and image quality deteriorates.
This sort of phenomenon is not limited to the case in which displacement of the paper transport position occurs; it may also occur under a condition in which an image forming operation is performed for paper (B5-size paper, for example) that has a comparatively short width dimension (dimension in the direction perpendicular to the paper transport direction) and overcharged toner affixes to the region outside this paper size, and when an image forming operation is performed for paper that has a comparatively long width dimension (A4 size paper, for example). That is, a fogged image is formed in the region corresponding to the difference in the width dimension of paper that has a comparatively long width dimension and paper that has a comparatively short width dimension.
As a method for eliminating the occurrence of this sort of fog phenomenon, image forming apparatuses have been proposed that perform the transfer process, not with a constant current control, but with a voltage control such that the transfer voltage is constant (for example, see JP 2002-49184A; hereafter referred to as “Patent Document 1”). Image forming apparatuses have also been proposed that reduce residual potential on the photosensitive body and make the transfer voltage constant, by de-electrifying the photosensitive body in an image forming apparatus that performs the transfer process with a constant current control (for example, see JP 2002-23576A; hereafter referred to as “Patent Document 2”). Further, it has also been proposed to avoid the generation of “fog” by decreasing the charging quantity of the photosensitive body in the non-image region below that of the image region as well as changing the development bias of the development unit (for example, see JP 2001-324843A; hereafter referred to as “Patent Document 3”).
However, when making the transfer voltage constant by the methods disclosed in the Patent Document 1 and the Patent Document 2, there is much damage to the photosensitive body, inviting a deterioration in the lifetime of the photosensitive body. That is, with the image forming apparatus disclosed in the Patent Document 2, because de-electrification of the photosensitive body is performed with a de-electrifying voltage of opposite polarity to the charging properties of the photosensitive body, it is possible that this will lead to a deterioration in the lifetime of the photosensitive body. Also, with the image forming apparatus disclosed in the Patent Document 1, a change occurs in the resistance value of the printing paper due to changing the environment of the apparatus for constant voltage control, and a change in the optimum voltage occurs. The voltage to the photosensitive body changes, the photosensitive body is damaged, leading in this case as well to a deterioration in the lifetime of the photosensitive body. Also, there is the problem that when controlling the transfer process at a constant voltage, transfer efficiency decreases in comparison to controlling the transfer process at a constant current.
With the technology disclosed in the Patent Document 3, because it is necessary to vary the charging quantity of the photosensitive body between the non-image region and the image region, multiple electricity sources for conferring an electric potential that differs for each respective region and a switching mechanism are necessary, and because this leads to complication of the configuration, the technology lacks applicability.
Further, with the technology disclosed in any of the Patent Documents 1 to 3, because a state is maintained in which overcharged toner is always affixed on the development sleeve, it is not possible to eliminate overcharged toner that causes the generation of a fogged image. Thus, because it is possible that a fogged image will be generated and a worsening of image quality will be invited when even a slight change in the charging potential of the surface of the photosensitive body occurs, this technology is still inadequate to realize a highly reliable image forming apparatus that can always provide a high quality image.